To begin with, in Hilchos Shabbos Siman 267 the Shulchan Aruch mentions the change in the blessing of Hashkivanu that we don't say 'Shomer Amo Yisroel' because as the Mishna Berura points out Shabbos itself is a guardian.
However in the next Siman the Shulchan Aruch introduces the blessing of 'Mein Sheva', which the Mishna Berura gives an explanation as to why we say it. The reason being that since majority of the Synagogues back in the day were located in a field, this location was more prone to Mazikim/Dangerous forces. But why did they need to, since it was Shabbos? The Shabbos itself is a guardian so why is they any need for the institution of the blessing of 'Mein Sheva'?